1. Field of the Invention
The present invention relates to a conductive roller and more particularly to a conductive roller, having a toner transport part, which is used as a developing roller, a cleaning roller, a charging roller, a transfer roller, and the like to be mounted on an electrophotographic apparatus.
2. Description of the Related Art
In the printing technique using an electrophotographic method, improvements have been made to satisfy demands for a high-speed printing operation, formation of a high-quality image, formation of a color image, and miniaturization of an image-forming apparatus. Toner holds the key to these improvements. To satisfy the above-described demands, it is necessary to form finely divided toner particles, make the diameters of the toner particles uniform, and make the toner particles spherical.
Regarding the technique of forming the finely divided toner particles, toner having a diameter not more than 10 μm and not more than 5 μm have been developed recently. Regarding the technique of making the toner spherical, toner having not less than 99% in its sphericity has been developed. To form the high-quality image, polymerized toner has come to be widely used instead of pulverized toner conventionally used. The polymerized toner allows the reproducibility of dots to be excellent in obtaining digital information as a printed sheet and hence a high-quality printed sheet to be obtained.
In compliance with the improvement in the technique of forming the finely divided toner particles, making the diameters of the toner particles uniform, making the toner particles spherical, and the shift from the pulverized toner to the polymerized toner, a conductive roller is especially useful as a developing roller which imparts a high charging property to toner and is capable of efficiently transporting the toner to a photosensitive drum in an image-forming mechanism of an electrophotographic apparatus such as a laser beam printer, and the like. Users demand that the high-performance function of the conductive roller is maintained to the end of the life of a product.
To comply with the above-described demand, the present applicant proposed a semi-conductive rubber member, as described in Japanese Patent Application Laid-Open No. 2007-286236 (patent document 1). The semi-conductive rubber roller is composed of at least two layers consisting of the surface layer, having a high electric resistance, which is made of the rubber composition and the base layer, having a low electric resistance, which is made of the electroconductive rubber composition. The semi-conductive rubber member provides a preferable charging characteristic in a favorable balance between the electric resistance value of the surface layer and that of the base layer.
It is difficult and necessary to make the thickness of both layers highly accurate. To achieve a high thickness accuracy for both layers, troublesome management is required, and the cost for producing the semi-conductive rubber member is high because the yield is low even though the troublesome management is made. Thus there is room for improvement in producing the semi-conductive rubber member at a low cost by a simple process management.
The present applicant also proposed a semi-conductive rubber member, as described in Japanese Patent Application Laid-Open No. 2006-99036 (patent document 2). The semi-conductive rubber member has the chloroprene rubber-containing conductive rubber layer composing the outermost layer and having a dielectric loss tangent of 0.1 to 1.8. The semi-conductive rubber member is capable of imparting a very high electric charge to toner or the like which sticks thereto and preventing the leak of an electric charge imparted to the toner.
In the semi-conductive rubber member, the dielectric loss tangent is adjusted to the above-described range by using the weakly conductive carbon black capable of imparting dielectric property thereto irrespective of the kind of the rubber component and without adversely affecting the electric resistance value thereof. Thereby the initial image density and durability (stability of charged amount of tone with age) are improved at a very high level. But there is room for improvement in achieving the initial image density and durability at the same time.
The present applicant also proposed a conductive rubber roller, as described in Japanese Patent Application Laid-Open No. 2004-170845 (patent document 3). The conductive roller is composed of the ionic-conductive rubber, having a uniform electrical characteristic, which contains the dielectric loss tangent-adjusting filler for adjusting the dielectric loss tangent thereof to 0.1 to 1.5. The conductive rubber roller is capable of imparting a proper and high charging property to toner, thereby providing a high-quality initial image. In the conductive rubber roller, the charged amount of the toner little decreases even after printing of a considerable number of sheets finishes. Consequently the conductive rubber roller keeps providing a high-quality image for a long time.
As disclosed in the patent document 3, the rubber component represented by epichlorohydrin rubber which contains chlorine atoms is used for the conductive rubber roller to allow the conductive rubber roller to be ionic-conductive. In this case, the rubber component containing the chlorine atoms has a high surface free energy. Thus toner as well as an additive for the toner are liable to adhere to the rubber component containing the chlorine atoms.
When ionic-conductive ethylene oxide monomer is polymerized, the conductive roller has a high surface free energy and is liable to get wet. Thereby the adhesion of the toner to the conductive rubber roller becomes high.
When an oxide film is formed on the surface of the conductive rubber roller by irradiating the surface thereof with ultraviolet rays or exposing it to ozone, the oxygen concentration of the surface of the conductive rubber roller becomes high. Thus the surface free energy increases. Thereby the adhesion of the toner to the conductive rubber roller further increases.
When the dielectric loss tangent of the conductive rubber roller is set to 0.1 to 1.5, it is possible to improve the charging property of the toner and hence decrease the toner transport amount. Thus the conductive rubber roller provides a high-quality image such as a half-tone image. On the other hand, the amount of the toner deposited on a developing roller decreases. Thereby when the conductive rubber roller is used as the developing roller, the adhesion of the toner to the developing rubber roller further increases.
The toner which has adhered to the conductive rubber roller does not considerably affect images formed in an early stage and images successively printed. But when images are printed in the following conditions (1) through (4), the influence of the toner that has adhered to the conductive rubber roller cannot be ignored. For example, normally, charged toner is transported to a photosensitive drum having an opposite electric charge by an electrostatic force (Coulomb force). But the transport of the toner by the electrostatic force is prevented because the adhesion of the toner to the developing roller is high. Thus there arises a problem that the print density becomes low, although the amount of charging applied to the toner does not change.
(1) When printing is made on a considerable number of sheets of paper and hence toner has affinity for the developing roller (for example, when image is printed at 1% on about 2,000 sheets of paper).
(2) When an average particle diameter of toner is not more than 8 μm and particularly not more than 6 μm.
(3) When printing is made not successively, but is suspended and made the next day.
(4) When the developing roller is used in a low-temperature and low-humidity environment in which the charged amount of toner is comparatively large.
Disclosed in Japanese Patent Application Laid-Open No. 2005-225969 (patent document 4) is the semi-conductive rubber member composed of the ionic-conductive rubber component containing the rubber having the polyether bond. Wax is added to the rubber component of the ionic-conductive rubber to decrease the free energy of the surface thereof so that the additive for toner can be prevented from adhering to the surface thereof for a long time. Further the semi-conductive rubber member is excellent in its processability and is capable of preventing the surface thereof from having defects such as molding nonuniformity and crack.
But when the semi-conductive rubber member is used as the developing roller, the adhesion of toner is still high, which may cause “decrease in print density”. In addition, there is a case in which the toner and a photosensitive drum are contaminated to a slight degree owing to the presence of a component having a low-molecular weight that leads to bleed of wax or the like and owing to the adhesion in environment having a comparatively high temperature of about 50° C. Therefore when the semi-conductive rubber member is used for a printer or the like demanded to provide a high-quality image, the kind of rubber or polymer which can be used for the semi-conductive rubber member is limited. Thus there is room for improvement in the semi-conductive rubber member.
When the above-described conductive roller is used as the developing roller, even though the charged toner has a state in which it is transported to a photosensitive drum having an opposite electric charge by an electrostatic force (Coulomb force), the transport of the toner by the electrostatic force is prevented because the physical adhesion of the toner to the developing roller is high. Thus although there is no change in the charged amount applied to the toner, print density drops, i.e., a problem of “drop of developing efficiency” occurs. As described above, the tendency that the developing efficiency drops although there is a large amount of the toner which can be transported is conspicuous in a high-speed printer having a speed of not less than 20 rpm.
As a result of the drop of the developing efficiency, a large amount of toner circulates in a toner box, which causes the toner to deteriorate. Consequently a decrease in the charged amount of the toner is accelerated. As a result, there occurs a problem that defective images are formed at an early stage of use. That is, when the developing roller transports a large amount of toner mainly because electrostatic and physical separation of the toner from the developing roller is unfavorable, most of toner transported by the developing roller does not contribute to printing to be made by the photosensitive drum, but remains in the developing roller and returns to the toner box. As a result, while the toner is repeatedly circulating within the toner box, the toner rubs each other and is damaged. In this way, the deterioration of the toner is accelerated to generate defective images at a later stage of an endurance use.
To solve the above-described problem, the present applicant proposed a semi-conductive rubber roller, as described in Japanese Patent Application Laid-Open No. 2007-72445 (patent document 5). The resin or the rubber of the semi-conductive roller essentially contains the resin or the rubber containing chlorine atoms and further contains 3 to 60 parts by mass of the titanium oxide for 100 parts by mass of the resin or the rubber.
In the semi-conductive roller, even in the case where the rubber component containing the chlorine atoms having a high surface free energy is used, it is possible to decrease the adhesion of the toner by adding a necessary amount of the titanium oxide to the resin or the rubber containing the chlorine atoms. In the concrete form of the patent document 5, the resin or the rubber containing chlorine atoms contains the agent such as weakly conductive carbon black for adjusting the dielectric loss tangent in addition to the titanium oxide so that the dielectric loss tangent in a predetermined condition is 0.1 to 1.8. In this manner, it is possible to apply a high charged amount to the toner and obtain a sufficient print density.
But in the form of the patent document 5, there is a case where the adhesion of the toner is so electrostatically high that the developing efficiency drops and the deterioration of the toner is accelerated. Consequently there is a fear that a defective image is generated at a later stage of an endurance use. Thus the semi-conductive roller of the patent document 5 has room for improvement in this respect.    Patent document 1: Japanese Patent Application Laid-Open No. 2007-286236    Japanese Patent document 2: Japanese Patent Application Laid-Open No. 2006-99036    Japanese Patent document 3: Japanese Patent Application Laid-Open No. 2004-170845    Japanese Patent document 4: Japanese Patent Application Laid-Open No. 2005-225969    Japanese Patent Application Laid-Open No. 2007-72445